The diagenesis of shell middens along the Gulf coasts of Texas and Florida

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Abstract

There are numerous subaerially exposed shell midden sites, the refuse piles of ancient peoples, located along the Gulf coasts of Texas and Florida. There is a proliferation of anthropological work associated with midden sites, but past geological work was supplementary. Most geologic research has focused on precipitated carbonate (caliche) and soil layers, not on the shells, themselves. This study expands upon the previous work completed on these sites in order to create an assessment of early, shallow burial diagenesis within shell middens. Using petrographic, mineralogical, and geochemical methods, in particular, thin-section petrography, X-ray diffraction (XRD), and radiocarbon dating corroborated via seriation, the mineralogical and structural changes midden shells and associated precipitates have undergone within these accumulations has been assessed and dated.
In all instances, diagenesis occurred rapidly, within several thousand years, and altered the physical and chemical structure of midden components. Hand sample observations revealed fractured and color-bleached mollusk shells. XRD work revealed the mineralogical conversion of aragonite to calcite within many of these shells. Brown discoloration, an indicator of diagenetic alteration, and dissolution were present in the shells made into thin sections. Several precipitates, notably a large amount of ankerite, were also discovered. There was no observable trend in terms of increasing or decreasing diagenesis with depth of burial.
The environmental conditions of the middens assessed for this work were comparable in terms of rainfall and marine exposure. A general trend of increasing diagenesis with age was observed in middens with comparable shell densities. It was the volume and density of carbonate materials, however, that seemed to be the most influential factor controlling the amount of diagenesis that took place.
With these results in mind, geologists and archaeologists must exercise caution in the future when using shells to obtain information about environment and age at the time of deposition, as those shells are likely to have been diagenetically altered. This study infers that, even in more recent systems, the data obtained are more likely to elucidate the influences and ages of introduced waters, and not the condition of the environment or time period at the occasion of shell deposition.